1. Field of the Invention
The present invention relates to ionophores which have the capacity to spontaneously assemble in solution. More particularly, the present invention concerns ionophore composed of hydrogen-bonded monomers of 5xe2x80x2-(t-butyl-dimethylsilyl)-2xe2x80x2,3xe2x80x2-O-isopyoidene-isoguanosine, or 5xe2x80x2-(t-butyl-dimethylsilyl)-2xe2x80x2,3xe2x80x2-O-isopropylidene-thio-isoguanosine, and use of these ionophores for removing 137-cesium ions (137Cs+) from nuclear waste.
2. Description of the Related Art
The United States faces a crisis in nuclear waste management. Safety and health issues surrounding nuclear waste have caused considerable interest in methods for separating radioactive isotopes4,5,9 Cesium-137 (137Cs+) is a major product from uranium fission. Due to its 30 year half-life, much of the 137Cs+ produced during the nuclear age still exists. 137Cs+ and 90Sr are the major heat sources in radioactive waste at Hanford Nuclear Reservation. Different methods for 137Cs+ removal and/or recovery from nuclear waste have been described, including selective precipitation as phospho-tungstate salts, inorganic ion exchanges, and solvent extraction using crown ethers.4,5 Since the Na+ concentration in nuclear waste is 103-104 times that of 137Cs+, ionophores with high Cs+/Na+ selectivity are required to separate 137Cs+ from nuclear waste.
There are few Cs+-selective ionophores known. Due to its large size, selective extraction of Cs+ (r=1.67 xc3x85) from solutions containing Na+ (r=0.97 xc3x85) and K+ (r=1.33 xc3x85) is challenging. Initially, design of Cs+ ionophores relied on large-ring crown ethers. Specifically, 21-c-7 and 23-c-8 have been proposed as ionophores for 137Cs+ extractions.6 The use of large-ring crown ethers poses special challenges because of their flexibility. Thus, both the Cs+ binding constants and Cs+ selectivities of large-ring crown ethers are relatively modest.10 Better results have been obtained using rigid macrocycles, such as calixarenes.7,12-14 While their Cs+ binding constants and Cs+/K+ selectivities are often impressive, these rigid ionophores have two major problems: 1) they are difficult to synthesize and are available only in small quantities; and 2) due to high association constants, cation recovery from the ionophore complex is difficult.
The present invention solves the problems noted above by providing an ionophore useful for removing 137Cs+ from nuclear waste. The term xe2x80x9cionophorexe2x80x9d as used herein refers to molecule or an assembly of molecules which either has the capacity to bind one or more ions, or is actually bound to one or more ions. The ionophore of the present invention comprises a plurality of monomers, wherein each monomer is noncovalently bound to another monomer, preferably through hydrogen bonding. Ideally, the ionophore is capable of spontaneously assembling in a solution containing the plurality of monomers. These inophores self-assemble at concentrations as low as 1 xcexcM in CHCl3 or CH3CN. The ionophore advantageously comprises identical monomers such as 5xe2x80x2-(t-butyl-dimethylsilyl)-2xe2x80x2,3xe2x80x2-O-isopropylidene-isoguanosine, 5xe2x80x2-(t-butyl-dimetlhylsilyl)-2xe2x80x2,3xe2x80x2-O-isopropylidene-thio-isoguanosine, or 2xe2x80x2,3xe2x80x2-Di-O-acetyl-5xe2x80x2-(t-butyl-dimethylsilyl)-isoguanosine. The ionophore may contain four or eight monomers. The ionophore is capable of forming a complex with an ion. This ion is preferably a cation such as Cs+, Na+, K+, Ag+, Hg+2, Pb+2, or Cd+2, preferably Cs+, and most preferably 137Cs+. Ideally, the binding affinity of the ionophore for Cs+ is greater than the binding affinity of the ionophore for Na+ or K+. The present invention includes the complex comprising the ionophore and the ion. Also, the present invention includes a micelle comprising a plurality of the ionophores, wherein each monomer comprises a hydrophobic moiety making micelle formation possible, wherein the ion is bound to one of the ionophores of the micelle; the hydrophobic moiety-containing monomer preferably has the following structure: 
wherein R is H or xe2x80x94SiRxe2x80x33, where Rxe2x80x3 is a hydrocarbon (CH2)m CH3 wherein m=0-22, or an ester C=O(CH2)nCH3, wherein n=0-22.
Additionally, the present invention is also directed to three methods:
(1) a method for forming the complex comprising the steps of adding the plurality of monomers to a solution containing the ion;
(2) a method for removing the ion from a solution comprising adding the plurality of monomers to the solution, and removing the resultant complex from the solution; and
(3) a method for removing the ion from an aqueous solution comprising adding a plurality of the hydophobic moiety-containing monomers to the solution. In method (3), the monomers form a composition comprising a micelle, wherein the micelle is composed of the ionophores, and the ionophores have ions bound thereto. The composition is then removed from the solution, preferably by ultrafiltration.